Air separation using air-rectification columns generally comprises a two-stage LINDE-FRANKL type of separation column in association with heat-exchanger means which can be of the reversing (REVEX) or regenerative (REGEN) type.
Basically, these systems operate by cooling an airstream drawn from the atmosphere in the heat exchanger (e.g. thereby removing high boiling point and high freezing point impurities such as water vapor and carbon dioxide), introducing the cold air into the bottom of a multistage high-pressure lower column of an air-rectification installation, carrying out a preliminary separation of air and oxygen in this high pressure stage of the column with refluxing so that oxygen is primarily drawn from the sump of the high-pressure stage and nitrogen is predominantly drawn from the top of the high-pressure stage, introducing this nitrogen and oxygen into a low-pressure upper stage of the column in which the top of the high-pressure stage, functions as part of a refluxing-boiler arrangement for the low-pressure upper stage, recovering nitrogen in a purity stage from the top of the low-pressure upper portion of the column, recovering oxygen from the lower portion of the upper low-pressure column, passing the product nitrogen and oxygen through the heat-exchanger means to provide the "cold" therein necessary to cool the aforementioned incoming gas stream, and drawing from the column a balance gas stream which is expanded, e.g. in an expansion turbine to provide a quantity of "cold" necessary to balance heat incursions into the system.
The balance stream can also be passed through the heat-exchanger means. Systems of this type and even more sophisticated air-rectification installations are described in PERRY'S CHEMICAL ENGINEERS' HANDBOOK (McGraw Hill Book Co., 1963, see especially page 28 of chapter 12) in which the LINDE double column is described in detail. Reference may also be made to PLANCK HANDBUCH DER KALTETECHNIK, 1957, l. Auflage, Volume VIII, pages 202, 203. Reference may also be made to the recently published discussion entitled "Large Air Separation Units Plants", AMERICAN SOCIETY OF MECHANICAL ENGINEERS 74-WAPID-8 in this connection.
In most systems for the separation of air into nitrogen and oxygen by two-stage low-temperature rectification using the LINDE double column mentioned above, the air to be rectified is cooled to the temperature necessary upon introduction into the lower or high-pressure stage of this column in a reversing main heat exchanger. Impurities such as water and carbon dioxide are frozen out of the air stream. By periodic interchange of the flow passages of the heat exchanger, e.g. by functional interchange of a pair of alternately effective heat exchanger sections or of a common heat exchanger or by the use of two interchangeable but separate heat exchanger units, a sparging gas is passed through the passages containing the frozen-out impurities which are evaporized and carried out of the apparatus with the sparging gas. The carbon dioxide, for example, is sublimated into the sparging gas.
For thermodynamic reasons, the re-evaporation of the frozen-out impurities require small-temperature differential between the incoming air and the sparging gas (see especially the aforementioned pages of "PLANCK'Handbook of Cold Techniques").
In order to maintain the small-temperature differentials in the main heat exchanger, it is necessary in many cases to provide the aforementioned balance stream which can comprise a gas drawn from the pressure stage of the LINDE double column, the balance stream being warmed in the cold part of the main heat exchanger in indirect heat-exchanging relationship with the incoming air.
To recover at least in part the mechanical energy of pressurization, the balance stream is usually expanded in an expansion turbine thereby converting the potential energy of pressurization of this gas into kinetic energy of work.
The "cold" resulting from the expansion of the gas is necessary in small installations to compensate for the insulation losses. In large air-rectification plants in which relatively small cold loss is encountered, the expansion produces more cold than is necessary to cover the losses. Because of the minimum insulation losses of large installations, the energy released is not effectively utilized, especially when one or more of the separation products is not required in a liquid state.
For the purposes of this description, reference will be made to the generation of "cold", the loss of "cold" and the like as is common in connection with low-temperature air separation. This is, of course, equivalent to the abstraction of heat, the incursion of heat into the system, etc.
In order to round out the state of the art with respect to two-stage air separation systems, it should be noted that it is also known to compress hot gases to be passed through the heat exchanger or warm gases emerging therefrom and even to compress cold oxygen within the network connected with the two-stage separating column. The compression of warm oxygen is intended to make this product available in a compressed state. The compression of warm air before it enters the heat exchanger is intended to provide the pressure subsequently needed at the high pressure stage of the LINDE double column and the compression of cold oxygen is intended to permit the latter to be used economically in the two-stage column where higher pressures are required.